Rice stand establishment for managing herbicide-resistant weeds

Research conducted by University of California, Davis (UC Davis) scientists could turn the tables to partially solve herbicide resistance management issues in establishing rice stands without reducing yields.

Herbicide resistance management is a gargantuan problem for rice growers impacted by fewer available herbicides and related modes of action or plant detoxification routes. Some herbicides including the workhorse Molinate are no longer available. California rice fields have a plethora of weeds including broadleaves, grasses, sedges, and others.

The weed science team led by Fischer is seeking solutions to minimizing weed populations in rice by experimenting with alternative stand establishment techniques. At the California Rice Experiment Station in Biggs, Calif., researchers conducted five alternate stand establishment trials about a quarter-acre in size each and replicated four times from 2004 to 2007.

Research objectives included diversifying weed species that emerge with rice through alternate aerobic and anaerobic seeding to create unfavorable conditions for certain weeds that built a resistance to available herbicides. Another objective was testing herbicide options to control herbicide-resistant weed biotypes including pendimethalin in drill seeding and glyphosate in stale seedbeds.

“We wanted to identify the ability of these systems to control herbicide-resistant weeds by diversifying management practices,” Fischer said.

About 95 percent of California’s rice crop is grown within 100 miles of the Sacramento Valley. The remaining acreage is located south of the Valley in the Delta.

Conventional water seeded: The aquatic, conventionally water-seeded rice ground included spring tilling followed by field flooding and seeding by airplane. Propanil was the most commonly applied herbicide.

Conventional drill-seeded: Spring tilling was followed by drilling into a dry seedbed, the method currently utilized in the Southern rice-growing states. The field was flooded at the four-leaf stage. The switch from traditional water seeding to dry seeding created a weed shift from aquatic weeds to grasses.

Spring tillage-stale seedbed-water seeded: The field was dominated by the weeds smallflower umbrella sedge, redstem, and ducksalad, but glyphosate (Roundup) used with the stale-seedbed technique provided good weed control. The grass seed bank decreased over four years.

No spring tillage-stale seedbed-water seeding: Spring tillage was replaced by tillage the previous fall that incorporated residue into the soil. Irrigation in the spring led to weed sprouting controlled with Roundup. Flooding and water seeding followed. No weeds existed during the growing season.

No spring tillage-stale seedbed-drill seeded: This aerobic system favored grass development which occurred along the drill furrows. Grasses were controlled with Roundup in the stale seedbed application, but other herbicides were also needed. No aquatic weeds emerged with the rice.

The field experiments proved that weed species change from water seeding to drill seeding; the aquatic weeds, and their resistant biotypes disappear.

Grass problems increase particularly with barnyardgrass and bearded sprangletop. If the problem is too severe, switching back to an aquatic system with no till and a stale seedbed is an option to reduce grass and resistant barnyardgrass infestations.

Rice yields were similar across the alternative stand systems. Fertilization was modified for water and dry seeding schedules.

In the fifth year, the researchers decided to make a switcheroo. Plots that were under four years of water seeding were no-till drilled following a Roundup stale seedbed or were moved back to the conventional water seeded system. Aerobically-established plots were converted to no-till water seeding after a Roundup-treated stale seedbed.

“The results were good stands of rice and few weeds without the use of herbicides in the water seeded treatments where stale seedbeds were involved,” Fischer surmised.

“This is a remarkable demonstration that growers can invest in a stale seedbed treatment to control resistant weeds with glyphosate or another similar compound for a brief period of time, and then return to the conventional rice production with the added bonus of dramatic reductions in weed control inputs that year,” Fischer said.

The single exception was rotating from a conventional water-seeded rice system into a drill-seeded, no-till application where the stale seedbed did not completely eliminate heavy grass infestations. “We advise against the continuous use of only drill seeding given the strong infestations that occur in this system and the opportunities for weedy red rice infestations,” Fischer said.

While drill seeding diminished aquatic weed pressure, the seeding practice should only be used in rotation with water-seeded rice using a stale seedbed treatment with glyphosate, Fischer says.

Future research will focus on farm implementation, predicting weed emergence in stale seedbed irrigation, and the long-term economic benefits to farmers.